With the arrival of an aging society, the number of care receivers is increasing in Japan. Elderly people with impaired motor function, especially those with disabilities in their lower body, need to travel by wheelchair. In addition, people disabled by birth or accident who have walking difficulty due to motor dysfunction also use wheelchairs as a necessity for life. Although some users operate wheelchairs by themselves, most of the users with severely impaired motor function have their wheelchairs operated by helpers and electric-powered wheelchairs have been developed in order to lighten the burden of helpers who take care of the abovementioned elderly or handicapped people. However, attention has been called to the problem that common electric-powered wheelchairs require a large space for turning and therefore it is difficult to use those wheelchairs in places which cannot offer sufficient space (e.g., ordinary houses and apartments).
On the other hand, in production facilities such as processing plants, a large amount of clear floor space is not available in order to secure space for installing machine tools, space for storing raw materials and so on. As is often the case, the floor space is sufficient for workers to walk around but not for transportation vehicles to move around with enough clearance. Therefore, in production facilities, a lot of transporting equipment such as conveyors is used, but members which are not suitable to be transported by conveyors (e.g., small component parts and machining swarf) are transported by transportation vehicles capable of moving around on the floor, so vehicles suitable for transportation in a small space have been desired.
Under these circumstances, vehicles capable of moving in all directions have been developed in order to make a change of direction even in a small space. Structures of such vehicles are roughly classified into two types: those which use wheels of particular kinds of structures and those which steer wheels. Examples of the former type of vehicle structure include one in which a plurality of rotors are attached to an outer peripheral ring constituting a wheel body (a portion corresponding to a rim) in a manner to be rotatable around the outer peripheral ring as an axis (see PTL 1) and one in which barrel-shaped split rollers are mounted on roller shafts which are provided in outer tangential directions of a wheel (see PTL 2).
The technique of the abovementioned structures employs, in addition to a wheel which plays a primary role, auxiliary wheels (rotors or barrel-shaped rollers) which are rotatable in perpendicular directions to an outer periphery of the wheel. Accordingly, the vehicle can move not only in back and forth directions but also in left and right directions. However, when the vehicle moves in diagonal directions, the auxiliary wheels do not rotate in the diagonal directions, but provide frictional resistance against the ground. This technique also involves a problem that the entire equipment is complicated in order to make a motor drive a wheel of such a complicated structure.
On the other hand, examples of the latter type of vehicle structure include one in which the direction of a wheel is changed by an output gear of a gyration shaft-driving motor, which engages with a gear provided on a gyration shaft in a roughly flat plate form (see PTL 3) and one in which a wheel support part for supporting a wheel is turned by a motor for gyration (see PTL 4).
PTL 3 is constructed such that only in a gyration mode, the output gear of the gyration shaft-driving motor comes in engagement with the gear of the gyration shaft and in an ordinary running mode the engagement is released and the driving force of the gyration shaft-driving motor is utilized for driving the wheel. According to this structure, however, the engagement and disengagement of the gear of the gyration shaft and the gear on the driving side are repeated and in the case of engagement, both the gears contact each other. As a result, in order to prevent loss of gear teeth, change of motor speed has been required (See paragraph 0010 of PTL 3). On the other hand, PTL 4 does not disclose switching between a gyration mode and a non-gyration mode in detail, but if the switching is executed by on and off of a motor, the frequency of using the gyration motor is low and operation rate of the motor cannot help being low.
In view of the above, the present inventors developed a mechanism for driving and steering a wheel by combining and redistributing outputs of two power sources by using a plurality of gear mechanisms (see PTL 5). This technique is a differential drive steering mechanism using a planetary gear mechanism, in which rotation of planetary gears sandwiched by a sun gear and a ring gear enables driving force to travel to be output to a wheel and at the same time movement of the abovementioned planetary gears around the sun gear enables the wheel to be steered.
This mechanism thus realizes axial rotation and orbital revolution of the planetary gears by simultaneously rotating the sun gear and the ring gear by two kinds of motors and controlling the number of rotations of these motors, so this is suitable as an omnidirectional moving mechanism. However, since the sun gear and the ring gear are widely different from each other in the number of teeth, the state of orbital revolution in differential drive may lack stability. In other words, in differential drive, it is possible to control the number of rotations of the ring gear having a greater number of teeth, and increase or decrease the number of rotations of the ring gear based on that of the sun gear. However, when both the gears rotate at high speed, the wheel has to be steered by decreasing the number of rotations of either one of the gears. When the number of rotations of the ring gear is to be decreased, delicate control of the number of rotations is possible in accordance with the number of teeth, but when the number of rotations of the sun gear is to be decreased, there is a possibility that a light change causes a wide change in differential state. There is also a possibility that the motors respectively driving the sun gear and the ring gear at different numbers of rotations are not stable in durability.
Another technique is constructed such that a pair of differential casters are formed by two parallel-arranged wheels and the direction of the casters is changed by the number of rotation of both the wheels (see PTL 6). In this technique, a pair of motors as power sources is respectively mounted in the casters, and when the motors rotate at the same number of rotations, the casters go straight and when the two motors rotate at different numbers of rotations, the casters make a change of direction. However, since these casters are constituted by two parallel-arranged wheels, a difference in frictional resistance acting on the two wheels and a difference in condition of a surface on which these casters move (a floor surface, a road surface, etc.) may affect whether desired impellent force can be obtained or not in accordance with the controlled number of rotations, so there is a need to be constituted by a single wheel.